Glastonbury Tor. The minaret spiralling up from the ruins of the Great Mosque at Samarra. The standing stones of Callanish. Sometimes, the most memorable architecture has little or nothing to do with the servicing and expression of matter-of-fact functions. It is often enigmatic and lonely structures rising from hilltops, desert sands or remote islands that possess the power to move us beyond words.

Theirs is the haunting architecture of myth, legend and spirituality. As we build ever more schlock, such powerful yet lonely designs on the imagination become ever more special.

Rising from prosperous, vine-growing farmland at Mechernich, a village some 30 miles south-west of Cologne, the newly completed Bruder Klaus Field Chapel is very special. Designed by the Swiss architect Peter Zumthor, it is a small, intensely spiritual shrine commissioned and built by a local farmer, Hermann Josef Scheidtweiler, in honour of "Brother Klaus", the religious mystic also known as Nicholas of Flue: a 15th-century hermit who is also the patron saint of Switzerland.

Scheidtweiler has created a building that will attract as many architects and their students as it will Catholic pilgrims. Yet, if Zumthor's elemental building causes visitors to be silent for just a while, it will have achieved at least one of its ends. Here is one of those physical still points in the turning world that, in an era of razzmatazz architecture, can be hard to find.

Brother Klaus (1417-87) was a farmer himself, but for the last 20 years of his life he was a hermit, surviving, according to legend, shoeless and hatless on a diet of the Holy Eucharist alone, though his son said he managed the occasional small piece of bread and some dried pears. In 1469, the local civil authorities built him a simple monastic cell and chapel. The most powerful politicians of the day came to seek his advice. Brother Klaus was declared a saint by Pius XII in 1947; his chapel and cell are still there, at Flüeli-Ranft, about 45 miles north-east of Lucerne.

The Zumthor chapel is a contemporary take on the hermit's chapel and cell. It takes the form of what appears to be a simple, handmade concrete tower, rising 12m in 24 tiers above the surrounding hills. The form of the building evokes the memory of Brother Klaus's teenage vision of the tower he would one day inhabit, so he believed, in the service of God. He had many visions, including, it is said, one he experienced in his mother's womb when he saw a star that lit up the world.

The apparently simple form of Zumthor's building proves to be far richer than it first appears. The concrete has been poured by Herr Scheidtweiler, family and friends, over a wigwam-like timber frame. Once the concrete had set, this frame was set on fire, creating walls inside the chapel that are strangely blackened and haunted with the ghosts of the timbers that once supported them. The floor is a frozen pool of molten lead, while the roof is open to the sky and, by night, the field of stars above. Rain and sunlight tumble and fall through this oculus to create atmospheric patterns of shade and glistening weather.

Zumthor's chapel is numinously dark inside, but when you look up, the oculus itself resembles the flare of a star - a reference, presumably, to Brother Klaus's vision in the womb. Being here alone is close to feeling, if not understanding, the faith that sustained the Swiss hermit.

So, here is a building containing just one room, with a roof that fails to keep out the rain, made of rough concrete, burned timber and lead. It has no electricity. No running water. No plumbing. No lavatories. No wind turbine. No solar panels. No air-con. No pictures hang on its walls. It offers no obvious, or accepted, sense of comfort. And yet it is compelling and very beautiful, offering solace. And this just 40 minutes away from Cologne, where Zumthor is building what promises to be a superb extension to the Kolumba art museum.

The Field Chapel is a project made in heaven for Peter Zumthor. Often presented as a bit of a recluse himself, he is no such thing; he just happens to like living and working in one of the most beautiful and remote parts of the country he was born in. He is, nevertheless, a master of beautifully crafted buildings for which the word "timeless" would appear to have been invented. He has the extraordinary knack of infusing his designs with a quality of space, light, texture and atmosphere that only religious buildings, at their best, tend to convey. The famous Thermal Baths (1996) he built at Vals, Switzerland, are as much a sensuous as a spiritual experience. This is modern architecture that appears to have grown, as if naturally, from the surrounding mountains. It is an extraordinary achievement by any standards.

For all his fame in architectural and connoisseur circles, Zumthor cannot be pressed to design buildings he has no real interest in. He has yet to design a commercial building and recently turned down the advances of a Dublin developer who decided he would like a house by the Swiss architect, and was happy to pay any price. This is not Zumthor's style. The son of a cabinet-maker, he likes to shape buildings "in their own time", and even though he has told me that would like to design a number of major civic buildings in Europe, he may or may not get around to doing so. Should he care? Not really. He is the counter-argument to the late Philip Johnson's witticism: "Architects are pretty much high-class whores."

As an architectural student in the late 1960s, Zumthor and his peers turned against formalistic design, believing architecture should be "natural" rather than deterministic and a slave to style. In his early professional years, he did more to preserve old Swiss buildings than to build new ones.

This ability to say no to commissions, to restrain from the impulse to build, has worked to his advantage. For Zumthor, each building he designs truly matters. Each is as much a product of a process of reduction as creation. Nearly all his buildings have that quality of being entire, and somehow exactly right: nothing - not the slightest detail - could be added or taken away from them without their being spoiled.